Don't know if I would be putting synthetic in. You don't know the history of that engine; if it's had irregular oil changes with OR without synthetic you could loosen up a bunch of ick too soon and clog your oil pump screen, starving your engine of oil.
Personally, I'd run 10w-30 Havoline (biased to Havoline) with several short oil/filter changes, say 1-2k at most; then go to synthetic if you wish, but do NOT extend the oil changes for more then 3k miles.
These engines like REALLY clean oil, whether it's regular or synthetic - in which, it's almost not worth running synthetic since any regular oil can run 3k just fine.
Synthetic also has the potential of finding weak seals/gaskets, and can cause oil leaks on old seals IF they're not used to synthetic oil.
If the dealer has all previous records - doubtful, and/or you are able to find some previous owner information in the glove box (gotten lucky before) and can call and see what kind of oil it has used in the past, it would make it easier for you to decide.
Otherwise, I personally would not be putting in true synthetic oil at that mileage with no history info to go from.
Just my .02 cents. I'm the original owner of my '97 3.2L engine Rodeo with every 3k oil/filter changes with Havoline mostly - and after spending several months on a website designated around oils & filters that people have tried in their rigs, I honestly see no difference in better wear numbers or cleanliness in regards to people using synthetic versus regular oil.
What is EXTREMELY important in these engines is clean oil and check it often - some tend to be oil users; mainly an issue with the 98+yrs, but I've heard older Rodeo's doing the same.